A single-photon-avalanche diode (SPAD) is a solid-state photodetector that is sensitive to single photons. A single photon incident on the SPAD generates a photo-induced carrier such as an electron. Due to a relatively high voltage bias across the SPAD, this photo-induced carrier triggers an avalanche of secondary carriers to produce an avalanche current pulse. The single photon incident on the SPAD is detected through detection of the avalanche current pulse. The SPAD enables determination of the arrival time of the single photon with an accuracy of about 10-100 picoseconds. The intensity of light incident on a SPAD may be derived from the rate of avalanche current pulses or from the number of avalanche current pulses detected over a period of time. SPADs are generally more light-sensitive than conventional image sensors such as charge-coupled device (CCD) sensors and complementary metal-oxide-semiconductor (CMOS) image sensors. SPAD-based image sensors are also shot-noise limited and achieve good signal-to-noise ratios even at high frame rates. Therefore, SPAD-based image sensors may operate at frame rates higher than those of CCD and CMOS image sensors.
Superresolution imaging is the art of enhancing resolution of an imaging system. In geometrical superresolution imaging, the resolution of an image sensor is enhanced. For example, the resolution of an image sensor may be enhanced beyond the actual pixel resolution of the image sensor. In one method, a single low-resolution image of a scene is post-processed to generate a superresolution image. This method utilizes interpolation between individual pixels to generate information at a resolution greater than the pixel resolution. In another method, several spatially shifted low-resolution images of a scene are combined to generate the superresolution image.